Construction vehicles typified by tractors often adopt electronically controlled traveling equipment. Generally, such electronically controlled traveling equipment uses two clutches which are actuated by their respective hydraulic actuators for low-speed operation and high-speed operation, respectively. Hydraulic pressure is supplied to and discharged from these two actuators to selectively engage the clutches.
One example of such a hydraulic clutch system is shown on FIG. 4. This clutch system comprises clutch C.sub.1 and C.sub.2 adapted for low-speed operation and high-speed operation, respectively. These clutches C.sub.1 and C.sub.2 are engaged and disengaged by hydraulic actuators 31 and 32, respectively, which are equipped with springs 34 and 35, respectively, for biasing their respective clutches in the directions to disengage them. Each of the hydraulic actuators 31 and 32 consists of a single-acting type hydraulic cylinder.
A single solenoid-operated selector valve 37 which has four ports and can assume two positions is disposed so as to be surround by the hydraulic actuators 31, 32, and a hydraulic pump 38. The solenoid control portion of the valve 37 is energized and deenergized tio switch the oil passage between two channels communicating with the actuators 31 and 32, respectively.
In the past, a valve of simple construction has been used as the solenoid-operated selector valve 37. In particular, as shown in FIG. 4, when the solenoid control portion is not energized, connections P-A and B-T are made. When the control portion is energized, connections P-B and A-T are made. When oil under pressure is being supplied from the hydraulic pump 38 into the hydraulic actuator 31 of the low-speed clutch C.sub.1 and the oil held in the hydraulic actuator 32 of the high-speed clutch C.sub.2 is being returned into a tank 39, if the solenoid control portion of the selector valve 37 is engergized to switch the oil passage in the valve, for switching the clutch, then the oil inside the actuator 31 is quickly returned into the tank 39 via the route A-T. Thus, the low-speed clutch C.sub.1 is immediately disengaged. However, the high-speed clutch C.sub.2 is not engaged until the oil delivered from the pump 38 enters the narrowed oil chamber (the volume of this chamber is minimal at this time) on the side of the piston in the actuator 32, increases the pressure in this chamber, and the piston produces a given stroke.
This is because, in the prior art solenoid-operated selector valve 37, oil under pressure is supplied into the port B irrespective of the pressure inside the port A at the time of switching. Therefore, the two clutches are not exactly successively engaged because of the delay of operation described above. If the load applied when the vehicle is running is large, the speed of the vehicle is greatly reduced momentarily or the vehicle comes to a halt. Then, the high-speed clutch is engaged, so that the vehicle frequently starts violently.
One conceivable method of avoiding these undersirable phenomena consists in connecting a solenoid-operated proportional pressure-reducing valve between the ports A and B in FIG. 4 and controlling the changes in the pressures inside the ports A and B in accordance with a predetermined program. However, this method involves complicated control. In addition, the valve is made large and complex in structure. As a result, it is inevitable that the valve is expensive. For these reasons, this method has not been practical.